The present invention relates in general to a method and apparatus for detecting flames which discriminates between various types of flames, and between directly received and reflected flame radiation.
Flame detectors have been in use for many years in various types of fire detection systems. For example, the Kennedy Space Center employs hundreds of ultraviolet radiation responsive flame detectors at the Space Shuttle launch pads to detect the occurrence of hydrogen fires. These are necessary because hydrogen fires generate very little visible radiation, and are thus very hard to detect with a conventional optical detector. A serious problem with radiation responsive flame detectors is their tendency to generate false alarms in response to radiation received from nonflame sources, such as a hot object or the sun for example. Numerous techniques have therefore been employed in the past to reduce the occurrence of false alarms by providing means in the detectors for discriminating between flame and nonflame radiation sources. One such technique is to make the detector responsive only to a plurality of discrete wavelengths which are present in the radiation generated by the flame to be detected, but are not present in other types of radiation. For example, a hydrogen fire emits radiation which includes both ultraviolet (UV) and infrared (IR) wavelengths and these wavelengths can thus provide a wavelength "signature" for the hydrogen flame. Another technique for reducing the occurrence of false alarms triggered by nonflame sources is to use flicker detection circuitry which monitors radiation intensity variations over time, and can thereby discriminate between a flickering flame source and a relatively constant intensity source such as the sun or a hot object, for example.
Unfortunately, even with these types of discrimination techniques, the detectors employed at the launch pads of the Kennedy Space Center continue to be prone to troublesome false alarms. In large part, these are the result of radiation received from reflections of a large flare stack flame which is continuously lit during fueling of the Space Shuttle's external hydrogen fuel tank. This flame is so large, i.e., approximately 100 feet in length, that although the flame detectors are purposely not aimed directly at the flare stack flame, the detectors will occasionally "see" the stack flame, or a reflection thereof off of a reflective surface. Since the stack flame has the same wavelength signature as any other hydrogen flame, the flame detectors cannot discriminate the flare stack flame or its reflection from any other hydrogen flame. This problem has therefore created the need for a flame detector which can discriminate both between radiation received directly from a flame, and radiation received from a reflection of a flame, as well as between radiation from a large flame and radiation from a small flame.